Always draw by hand in pencil
Drawing by hand can be done without delay, while you are on the floor. As you draw you will think of further information that you need.
Drawing by hand means that you can do it yourself, which is key to understanding the material and information flows.
Drawing by hand means you will focus on understanding the flow, instead of on how to use the computer. The point of value stream mapping is not the map, but understanding the flow of information and material.
Having to manually fine-tune your drawings will improve your mapping ability. Keep an eraser handy